PROJECT SUMMARY Stroke is the leading cause of adult-onset disability; however, treatment of the locomotor dysfunction that usually results has been limited. Individuals who experience a stroke often continuously and inadvertently contract their muscles due to changes in how the brain coordinates movement, among other mechanisms. This extra muscle contraction causes unwanted resistance?known as increased joint stiffness and viscosity. Patients must fight against this resistance during locomotion, and consequently, locomotor function is severely impaired. To treat the resulting gait deficits, clinicians use coarse, qualitative assessments performed while the patient is relaxed and their leg is not bearing weight. Although treatments based on these assessments have had some success in reducing altered limb mechanics in resting conditions, these improvements have not extended to functional benefits in locomotion. Thus, current clinical practice is hindered by the coarse, qualitative nature of the assessment metrics, as well as the mismatch between passive assessment and the desired improvements in dynamic activities, such as walking. Despite much knowledge of joint stiffness during resting conditions, scientific and engineering challenges have prevented the study of these properties during gait, where they could be used to inform clinical treatment. Over the past several years, our group has overcome these challenges, pioneering a new approach to measure the stiffness and viscosity of the ankle dynamically during locomotion. We previously quantified how these properties vary during walking in able-bodied individuals, which led to the development of a new class of ankle prostheses. Although we have begun to study how ankle stiffness and viscosity change throughout walking in healthy individuals, there is a gap in our knowledge of how pathology alters these properties during locomotion, and how this knowledge can impact clinical treatment. Joint stiffness and viscosity are especially important in assessment of individuals post-stoke because these properties directly characterize the unwanted resistance that occurs. The specific objective of this proposal is to leverage our innovative approach to investigate how ankle joint stiffness and viscosity vary during walking in individuals post-stroke, and compare these data to existing assessments and age-matched control subjects. Our rationale is that knowledge of how these properties change throughout walking will provide new, fundamental information that can be used to understand, assess, and treat the functional aspects of altered joint mechanics. Our hypothesis is that joint stiffness and viscosity will be greater in individuals post-stroke, when compared to healthy, age-matched control subjects. This work will provide a new understanding of impaired walking mechanics, and provide a novel, quantitative method to dynamically assess and track the changes that occur following neural injury. Finally, the information from this study can be used to quantify the effect of future targeted physical and pharmacological interventions, as well as the development of novel assistive technologies.